The severe bone phenotype in microphthalmia (mi) mutant mice indicates that the helix-loop-helix (HLH)-zipper transcription factor encoded by the microphthalmia (Mi) gene has a significant role in the terminal differentiation of multi-nucleated osteoclasts. In addition to providing a very interesting system to study developmentally regulated gene expression in a mammalian system, studying the Mi gene may have direct applications to significant human diseases. In particular, osteoporosis in post-menopausal women and the osteolytic bone destruction and hypercalcemia that occurs in patients with multiple myeloma are examples of clinical conditions where this research may have potential impact. As the knowledge concerning transcriptional regulation and signal transduction increases at exponential rates, new pharmacological targets and strategies for interfering selectively with processes that contribute to human disease become possible. The biology of the Mi locus suggests that the product encoded by this gene might provide such opportunities for the treatment of human bone disease. The long term goal of this application is to determine, at the molecular level, the function of the Mi protein in normal osteoclast biology. The specific aims of this proposal are: 1. To identify and characterize cis-elements in identified target genes regulated by the Mi protein in the osteoclast cell lineages. 2. To identify the trans-requirements of Mi action in osteoclasts, both through structure function analysis of the Mi gene, but also by identifying osteoclast-specific partners for Mi action. 3. To determine how the Mi product is negatively regulated by CSF-1/c-fms tyrosine kinase signaling pathways in myeloid cell lines and to determine if this regulation occurs in the osteoclast. These studies will be performed in three experimental systems: heterologous cell lines, primary osteoclast-like cells cultured in vitro, and in vivo in mouse transgenic and genetic models. Such information will suggest strategies to specifically interrupt Mi function in the osteoclast.